1. Field of the Invention
The present invention relates to determining the location of remotely emplaced objects, and in particular, to methods and devices for determining the location of remotely emplaced objects, such as munitions.
2. Prior Art
Geolocation and navigation in GPS denied environments, particularly indoors, has attracted the interest of researchers from very diverse fields. Many of the contemporary technologies are based on trilateration, triangulation, received signal strength, signal-of-opportunity, and frequency of arrival. The latter requires relative motion between the target and the known source. Trilateration requires highly accurate clocks and time synchronization, while triangulation requires precise knowledge of the distance between two sources and either distances or angles from these two sources to the target.
Pseudolites (ground-based pseudo-satellite transmitters) system was developed as a complementary technology to enhance the location resolution of aircraft on landing approach. Initially developed as an Integrity Beacon Landing System (IBLS), it is now being considered as a ground-based GPS equivalent to provide accurate location of a target. While this system is building on a mature technology, however, it inherits its shortcomings. In particular, the use of high power beacons is undesirable in a battlefield environment as it is immediately visible to an enemy. Furthermore, to provide the prerequisite time synchronization, the pseudolites are typically connected using wired technologies. Additionally, pseudolites require multiple transmitters positioned far enough apart to achieve the required remotely emplaced munitions (EM) geolocation accuracy.
Active or passive RFID positioning technologies are being incorporated into diverse environments, from the indoor office, to warehouse and outdoor (ship yards), primarily for tracking inventory movement. Geolocation information of moving or stationary targets requires tag readers which are located at geographically defined positions. While this technology is proving useful in a controlled environment, its utilization for geolocation of EMs is fraught with difficulties. It represents a medium power, short distance solution, typically requiring a large number of reader tags for accurate positioning. RFID readers and tags are omnidirectional and thus require considerably more power during transmission compared with directional systems.
The 3D-ID system is envisioned as a GPS equivalent for a geographically defined local area that uses inexpensive tags. It has its own antenna infra-structure, currently developed for indoor applications, which can be deployed in outdoor environments. The system is organized into cells, which are managed by a controller, with up to 16 antennas connected by coax cables. The cell controller quickly cycles among antennas, determining distances to whichever of them are in range of the tag. The tag's location is determined from a minimum of three antenna fixes. Geolocation is based on the existence of digital maps of the building or the infrastructure. For the outdoor environment, coax cables could be replaced by a wireless link, but the technology is short range and heavily dependent on a large number of readers, which are essentially relatively high power and omnidirectional beacons, thereby making it unsuitable for many applications, including in the battlefield.
A Q-track system uses low frequency and near field electromagnetic ranging and has been demonstrated for short distance, wireless ground sensor network. Self-powered ad hoc network (SPAN) is a mesh network of self-organizing, self-healing sensors. SPAN systems, using low-frequency tags operating at 1 MHz, provide real time location resolution of 1 m with a maximum range of up to 25 m. These systems are primarily for indoor environments. In addition to the shortcoming of short operating distances, the RFID tags are typically large (e.g., 5.1 cubic inch). There does not appear to be a practical path to adaptation of this technology for EM geolocation application.
Wireless Local Area Network (WLAN), commonly known as Wi-Fi, is a widespread Radio Access Technology (RAT) used in wireless networks, such as, Blue tooth and Zig-Bee. Wi-Fi positioning techniques are based on a number of key technologies including Cell-ID, time, angle and RSSI.
Wireless Wi-Fi sensor networks (WSN) suffer from the lack of localization technologies. Localization is the process through which motes (a sensor network device) in a network are associated with their physical location rather than a network address. Self-localization of wireless motes is an enabling technology for both very large networks and for networks with movable motes. Automatic location discovery is also critical for “sprinkle deployments” desired for military and environmental monitoring applications. For many of these applications, cooperative localization between individual motes, rather than with base stations, is used to establish a mesh network. Deployment of wireless sensor node technology has some serious concerns in a non-cooperative battlefield environment. In particular, the geolocation of EMs will be based on the relative location of other EMs, potentially leading to large positional uncertainties. Main shortcoming of the WSN technology is the requirement of large number of widely dispersed base nodes outside as well as inside the deployment zone, thereby making them impractical for many applications, including in the battlefield.
UAV SAR based systems (or radar) capture the image of the region of interest based on back scattered signal. The most serious shortcoming of radar based technologies is the inability to discriminate the emplaced munitions from other similar sized objects on the ground. Other shortcomings include long set-up time and unacceptably long latency.
Acoustic (or ultrasonic) positioning systems have been used in mines and many indoor positioning applications. This approach relies on a small set of fixed anchors and a set of closely spaced beacons that can guarantee LOS transmission between the beacons and the target to be located. Time of flight measurements are used to determine distance, leading to location. These systems have a very limited range and accuracy and are very susceptible to countermeasures.
Laser based ranging and other methods relying on visual imaging do not present acceptable solutions since they cannot be operated in all-weather conditions and at night.
A number of direction finding (DF) methods have been developed for an object determining its general direction (bearing) relative to a beacon. Methods such as the so-called direction of arrival (DOA) or angle of arrival (AOA) measurement techniques have been well developed to provide such direction indicating information. The current technologies for DOA (or LOB) are basically crude indicators of the object's bearing and do not provide precise angle measuring systems. Thus, these methods do not provide the means for accurate geolocation.
Since the inception of RADAR, hardware associated with DF has seen some evolutionary changes. Most of the recent advances have been in the development of new algorithms, such as MUSIC, which can find directions of multiple objects. The physics behind DF and beam-forming has essentially remained unchanged. Early DF techniques were based on manual rotation of directional receiving antennas, surprisingly, these are still in use. Automated DF systems, based on four (or eight) symmetrically placed antennas on a rotating platform exploit the Doppler shift to measure the direction of the incoming signal. Watson-Watt designed an Adcock antenna, which has four equally spaced vertical elements, in E-W and N-S configuration, to compute the line of bearing (LOB) of an incoming signal. The Adcock pairs can be implemented using crossed ferrite loops, crossed dipole elements, crossed monopole elements and crossed loops. The fundamental drawbacks of the technology are the narrow operational frequency band-width, low angular resolution and the large antennas make it impractical for accurate geolocation.
Another method for LOB measurement uses linear (or circular) phased arrays, which require extensive computational resources. The length of the array determines the angular resolution. As a simple example, an angular resolution of one degree, at a frequency of 10 GHz, requires a linear array with a width of 24 meters. Clearly, such systems do not represent a practical solution for accurate geolocation.
Other antenna structures, known as smart antennas, are used in wireless communication systems, atop cellular towers. These antennas are arranged in a cluster to provide tracking coverage of users in range. Typically, the antennas are switched to track the object as it moves from one bearing sector to the next. For example, a six cluster antenna system gives a LOB width of 60 degrees. Such smart antennas have serious angular resolution shortcomings and are impractical for accurate geolocation.